1. Field of the Invention
The present invention relates to an illumination device and to an electrooptical device having the illumination device. More particularly, the present invention relates to a structure of an illumination device which may be suitably used as a front light which is placed in front of a display surface of an electrooptical device so as to illuminate the display surface in dark environments and to allow the display surface to be visible therethrough in a lighted environment.
2. Description of Related Art
Conventionally, reflective liquid crystal display panels that do not consume large amounts of power are used as electrooptical devices for use in display sections of portable devices. However, the displays are not readily visible in dim conditions, such as during the night. In contrast, since transmissive liquid crystal display panels have backlights, displays thereof are readily visible even under dim conditions. However, the backlights consume large amounts of power, and the displays are rather difficult to view outdoors in the daytime since the environment is bright.
In order to solve the above problems, a liquid crystal display device having a front light serving as a surface light-emitting element has been proposed in which a light guide plate is placed in front of a reflective liquid crystal display panel, light from a light source, such as a cold cathode-ray tube, placed adjacent to the end of the light guide plate, is introduced into the light guide plate, and the light is emitted from the surface of the light guide plate toward the liquid crystal display panel, thereby allowing the display to be visible even in a dim environment. In a liquid crystal display device having a front light, since the liquid crystal display panel is visible through the light guide plate in the daytime, it can be used as a normal reflective liquid crystal display panel. In a dim environment, the liquid crystal display panel is illuminated by lighting the front light, so that the display is visible through the light guide plate.
FIG. 11 schematically shows the general plan structure of a front light 20 for a conventional liquid crystal panel, and FIG. 12 is an enlarged general sectional view of the front light 20. The front light 20 may consist of a light source 21 formed of a cold cathode-ray tube or the like, a platelike light guide plate 22 made of a material having a high refractive index, such as acrylic resin, so as to introduce light from the light source 21 through an end face 22d and to guide the light to the right side in the figure, and a reflector 23 placed to surround the light source 21. On the surface of the light guide plate 22, gently inclined sections 22a inclined at a slight angle and steeply inclined sections 22b inclined at an angle greater than that of the gently inclined sections 22a are cyclically arranged to the right side in the figure. Cuneal projections formed by the gently inclined sections 22a and the steeply inclined sections 22b are formed in stripes so as to extend in the direction of the plane of the drawing paper. In contrast, a back surface 22c of the light guide plate 22 is flat.
When light is introduced from the light source 21 into the light guide plate 22, it is totally reflected by the inner surface of the light guide plate 22, and travels to the right side in the figure. When the light is totally reflected by the steeply inclined sections 22b, it is directed toward the back surface 22c, and is emitted as illumination light 22A from the back surface 22c in the downward direction in the figure. For this reason, light can be emitted downward from the surface of the light guide plate 22. When the light guide plate 22 is viewed from above, since it is made of a transparent material, a display formed therebelow is visible therethrough.
While light emitted from the light source 21 is emitted downward by using total reflection at the steeply inclined sections 22b in the above-described front light 20, it is not totally reflected, depending on the angle of light incident on the steeply inclined section 22b, and leakage light 22B leaks above the light guide plate 22, as is shown in the figure. Since the steeply inclined sections 22b are formed like bands in a direction orthogonal to the travel direction of propagating light that propagates inside
In contrast, in some devices, the direction, in which the gently inclined sections 22a and the steeply inclined sections 22b are alternately arranged, is placed obliquely with respect to the principal viewing direction F, as shown by dotted-chain lines in FIG. 11. By doing this, since the intensity peak direction of the leakage light 22B is deviated from the principal viewing direction F, as shown by dotted-chain lines in FIGS. 9A-B and 10, visibility is improved. In this method, however, since the brightness peak direction of the display is also deviated with the deviation of the intensity peak of the leakage light 22B, the display is somewhat dimmed. Moreover, when viewed from the deviated direction, the deviated direction coincides with the intensity peak of the leakage light 22B, which rapidly deteriorates visibility.
Accordingly, the present invention aims to solve the above problems, and an object of the present invention is to prevent visibility from being deteriorated by controlling the leaking direction of leakage light emitted from the surface. of a light guide plate, and to thereby improve the performance as an illumination device.
In an exemplary embodiment of the present invention, an illumination device includes a light source, and a light guide plate for introducing light emitted from the light source from an end face and emitting the light from one surface thereof, wherein the other surface of the light guide plate opposing the one surface has a reflective face section for reflecting the light from the light source toward the one surface or emitting the light from the other surface according to the incident angle of the light, the reflective face section includes at least first and second faces, and the emitting direction of the light emitted from the first face and the emitting direction of the light emitted from the second face differ from each other. Herein, the light emitting direction means an emitting direction in which the light intensity is highest.
According to the illumination device of the present invention, even when leakage light leaks from the reflective face section toward the viewer, since the reflective face section includes the first and second faces and the first and second faces emit leakage light at different emitting angles, the leakage light is not collected in one direction, but is dispersed. Therefore, it is possible to prevent visibility from being deteriorated due to leakage light.
It is preferable to set the formation angles of the first and second faces so that leakage light is emitted from the first. and second faces in directions different from the principal viewing direction. By doing this, it is possible to prevent a large amount of leakage light from being emitted in a direction identical to or close to the principal viewing direction, and to thereby improve visibility when viewed from the principal viewing direction.
In another embodiment of the illumination device of the present invention, an angle, formed by the emitting direction of light projected onto the surface of the light guide plate and emitted from the first face and the emitting direction of light projected onto the surface of the light guide plate and emitted from the second face, is between 0xc2x0 and 180xc2x0.
According to this embodiment, even when the azimuth angle, at which the viewer views the light guide plate, is changed, since the first and second faces emit leakage light at different emitting angles, the leakage light is not collected in one direction, but is dispersed, which can prevent visibility from being deteriorated due to leakage light.
In a further embodiment of the illumination device of the present invention, the first face and the second face are formed alternately and continuously. For example, it is preferable that a plurality of reflective face sections be formed cyclically and repeatedly. In a case in which the reflective face sections are curved surfaces, it is preferable that the curved face sections be formed in a cyclic form. More particularly, it is preferable that the curved face sections be formed at sufficiently short intervals. This prevents brightness of illumination light and transmitted light from varying, and facilitates the production of the light guide plate.
High polymeric material having a high reflective index, such as acrylic resin, may be suitably used as the light guide plate used in the illumination device of the present invention. It is preferable that the reflective face sections be formed of inclined surfaces disposed at a predetermined angle to the plane parallel to one surface of the light guide plate.
An electrooptical device of another exemplary embodiment of the present invention includes an electrooptical element having an electrooptical layer disposed between a pair of substrates and a reflective film disposed on one side of the electrooptical layer; and an illumination device placed on the viewer side of the electrooptical element, and including a light source and a light guide plate for introducing light emitted from the light source from an end face and mainly emitting the light toward the electrooptical element, wherein a surface of the light guide plate on the viewer side is provided with a reflective face section for reflecting the light emitted from the light source toward the electrooptical element or emitting the light from the viewer-side surface according to the incident angle of the light, and a transmissive face section for allowing the electrooptical element to be visible therethrough from the viewer side, the reflective face section includes first and second faces, and the emitting direction of the light emitted from the first face and the emitting direction of the light emitted from the second face are different.
As the electrooptical element used in the electrooptical device of the present invention, a liquid crystal display device may be suitably used which uses a liquid crystal layer, such as an STN liquid crystal layer, an ECB liquid crystal layer, or a TN liquid crystal layer, as the electrooptical element.
According to the illumination device of another exemplary embodiment of the present invention, even when leakage light leaks from the reflective face section toward the viewer side, since the reflective face section includes the first and second faces and the first and second faces emit the leakage light at different emitting angles, the leakage light is not collected in one direction, but is dispersed, which can prevent visibility from being deteriorated due to leakage light.
It is preferable to set the formation angles of the first face and the second face so that the emitting directions of leakage light from the first face and the second face are different from the principal viewing direction. By doing this, a large amount of leakage light is prevented from being emitted in the direction identical to or close to the principal viewing direction, and visibility when viewed from the principal viewing direction can be improved.
In another embodiment of the electrooptical device of the present invention, an angle, formed by the emitting direction of light projected on the surface of the light guide plate and emitted from the first face and the emitting direction of light projected on the surface of the light guide plate and emitted from the second face, is between 0xc2x0 and 180xc2x0.
According to this embodiment, even when the azimuth angle, at which the viewer views the electrooptical device, is changed, since the first and second faces emit leakage light at different emitting angles, the leakage light is not collected in one direction, but is dispersed, which prevents visibility from being deteriorated due to leakage light.
In a further embodiment of the electrooptical device of the present invention, the first face and the second face are formed alternately and continuously. For example, it is preferable that a plurality of reflective face sections be formed cyclically and repeatedly. In a case in which the reflective face sections are curved surfaces, it is preferable that the curved face sections be formed in a cyclic form. More particularly, it is preferable that the curved face sections be formed at sufficiently short intervals. This prevents brightness of illumination light and transmitted light from varying, and facilitates the production of the light guide plate.
A high polymeric material having a high refractive index, such as acrylic resin, may be suitably used as the light guide plate used in the electrooptical device of the present invention. It is preferable that the reflective face sections be formed of inclined surfaces disposed at a predetermined angle to the plane parallel to one surface of the light guide plate.
An electronic equipment of another exemplary embodiment of the present invention includes an electrooptical device as a display section, and an input device for supplying an image signal to the electrooptical device, wherein the electrooptical device includes an electrooptical element having an electrooptical layer disposed between a pair of substrates and a reflective film disposed on one side of the electrooptical layer, and an illumination device placed on the viewer side of the electrooptical element, and including a light source and a light guide plate for introducing light emitted from the light source from an end face and mainly emitting the light toward the electrooptical element, and wherein a surface of the light guide plate on the viewer side is provided with a reflective face section for reflecting the light emitted from the light source toward the electrooptical element or emitting the light from the viewer-side surface according to the incident angle of the light, and a transmissive face section for allowing the electrooptical element to be visible therethrough from the viewer side, the reflective face section includes first and second faces, and the emitting direction of the light emitted from the first face and the emitting direction of the light emitted from the second face are different.